1) Field of the Invention
The present invention relates to an optical waveguide device suitable for use with optical communication.
2) Description of the Related Art
An optical waveguide device having a substrate made of electro-optical crystal of lithium niobate (LiNbO3, hereinafter referred to simply as LN), LiTaO2 or the like is formed by forming a metal film on part of a crystal substrate and then carrying out thermal diffusion or by forming an optical waveguide by means of proton exchange in benzoic acid or the like after patterning and then providing an electrode in the proximity of the optical waveguide. Then, an electrode for providing refraction index variation to light propagating in the optical waveguide is formed on the electro-optical crystal on which such an optical waveguide as just described is formed so that an optical device for performing optical modulation can be formed.
An electro-optical modulator in which a ferroelectrics material such as LN described above is used has been placed into practical use already in an optical communication system and so forth, and also marketing of a high-speed optical modulator which can modulate an optical wave with a high-frequency electric signal, for example, of approximately 40 GHz is advancing.
FIG. 12 is a schematic top plan view showing, as an example of the high-speed optical modulator described above, a clock modulator 100 used in a CS-RZ (Carrier Suppressed Return to Zero) modulation method or the like. FIG. 14 is a schematic top plan view showing a CS-RZ optical modulator 110 to which the clock modulator 100 shown in FIG. 12 is applied.
The clock modulator 100 shown in FIG. 12 includes a Z-cut substrate (LN substrate) 101 formed, for example, from lithium niobate which is electro-optical crystal, an optical waveguide device 100A formed on the LN substrate 101 and including a Mach-Zehnder type optical waveguide 102, a signal electrode 103 and ground electrodes 104, and a clock signal source 105 for generating a (high speed) clock signal to be supplied from the upstream side of the signal electrode 103 in a light propagation direction.
The Mach-Zehnder type optical waveguide 102 includes, for example, an input optical waveguide 102a, two interference optical waveguides 102b and an output optical waveguide 102c formed integrally with each other and is formed as a coplanar electrode from the signal electrode 103 and the ground electrodes 104. Where the Z-cut substrate 101 is used as in the clock modulator 100 shown in FIG. 12, generally the signal electrode 103 and one of the ground electrodes 104 are disposed just above the interference optical waveguide 102b in order to utilize refraction index variation by an electric field in the z direction.
Further, while the signal electrode 103 and the ground electrodes 104 are patterned above the interference optical waveguide 102b as described above, a buffer layer not shown is interposed between the substrate 101 and the signal electrode 103 and ground electrodes 104 in order to prevent absorption of light propagating in the interference optical waveguide 102b by the signal electrode 103 and the ground electrodes 104. As the buffer layer, for example, a SiO2 layer having a thickness of 0.2 to 1 μm is used.
Where the clock modulator 100 configured in such a manner as described above is to be driven at a high speed, terminal ends of the signal electrode 103 and ground electrodes 104 are connected to each other through a resistor so as to form a traveling wave electrode, and a microwave signal is applied from the light input side of the traveling wave electrode. At this time, by electric field variation when an electric signal of microwaves is from the clock signal source 105 through the signal electrode 103 and the ground electrodes 104, the refraction indexes of the two interference optical waveguides 102b individually vary to +Δna and −Δnb.
Consequently, since the phase difference of light is varied by cyclical variation of the electric field in a process wherein CW (Continuous Wave) light inputted to the input optical waveguide 112a is branched into two lights and then the two lights propagate individually through the two interference optical waveguide 102b, intensity-modulated signal light is outputted from the output optical waveguide 102c. 
FIG. 13 is a view illustrating an optical modulation characteristic of the clock modulator 100 shown in FIG. 12 with respect to the driving voltage. In a Mach-Zehnder type optical modulator such as the clock modulator 100 as shown in FIG. 13, the light intensity has a characteristic that it varies in a sinusoidal wave with respect to the applied voltage (refer to reference character A in FIG. 13). If an electric signal of 20 GHz having the voltage amplitude (2 V π) in one cycle of the sinusoidal wave is applied (refer to reference character B in FIG. 13), then a clock optical signal having a variation cycle of optical output power of 40 GHz can be outputted (refer to reference character C in FIG. 13).
Further, the CS-RZ optical modulator 110 shown in FIG. 14 is configured by integrally forming a clock modulation section 111 equivalent to the clock modulator 100 described hereinabove with reference to FIG. 12 and a data modulation section 112 on the same substrate 101 through a curved and folded back waveguide 113. The data modulation section 112 is formed from a Mach-Zehnder type optical waveguide 114 connected to the curved and folded back waveguide 113, a signal electrode 115 for applying an NRZ data electric signal of 40 Gb/s, and ground electrodes 104 provided around the signal electrode 115. It is to be noted that a groove 113a is formed along an outer periphery of the curved and folded back waveguide 113 in order to promote a confining effect of light propagating in the curved and folded back waveguide 113.
In the CS-RZ optical modulator 110 configured in such a manner as described above, inputted CW light is modulated into a clock optical signal of 40 GHz with a clock electric signal of 20 GHz by the clock modulation section 100, and the clock optical signal of 40 GHz inputted through the curved and folded back waveguide 113 is modulated into an RZ optical signal of 40 Gb/s with an NRZ data electric signal of 40 Gb/s by the data modulation section 112 so that the modulated optical signal can be outputted.
It is to be noted that it is also known that an effective refraction index of a microwave can be controlled by varying the sectional shape of the electrodes 103 and 14 so that the speeds of light and the microwave are matched with each other to obtain an optical response characteristic in a broadband.
Further, techniques relating to the present invention are disclosed in Japanese Patent Laid-Open No. 2006-195256 (hereinafter referred to as Patent Document 1), International Publication No. 2005/008923 (hereinafter referred to as Patent Document 2), and Japanese Patent Laid-Open No. 2003-255283 (hereinafter referred to as Patent Document 3).
Patent Document 1 discloses a configuration wherein a 2×2 MMI (Multimode Interference) coupler is applied to a coupling portion of a Mach-Zehnder interferometer which forms a clock modulator in order to reduce the loss while the phases of two outputs of the MMI coupler whose phases are reverse from each other are made the same phase utilizing a difference between the radii of curved and folded back waveguides so as to be connected to a waveguide for data modulation as it is.
Meanwhile, FIG. 9 of Patent Document 2 discloses a configuration wherein two waveguides are formed on a planar optical waveguide circuit substrate made of LN or the like and are disposed near to each other in the proximity of the opposite ends thereof to individually form couplers on the upstream and downstream sides in a light propagation direction such that a delay difference is applied between two paths formed from the waveguides between the couplers utilizing double refraction index variation by an electric field.
Further, Patent Document 3 discloses a technique wherein modulation signals synchronized with each other are supplied individually to two Mach-Zehnder type optical modulators through signal transmission lines having lengths different from each other so that optical signals having a time difference corresponding to the difference between the lengths of the signal transmission lines are generated.
In the Mach-Zehnder type optical modulator, reduction of the amplitude of an electric signal to be applied to an electrode, that is, reduction of a driving voltage, is conventionally considered as a significant subject, and it is demanded to effectively supply an electric field for modulating light.
However, in a case wherein, for example, such a CS-RZ as described above is applied as a modulation method or in a like case, a driving voltage equal to twice V π is required. In this instance, a high driving voltage is required in comparison with that in an alternative case wherein a driving voltage equal to V π is used by a different driving method. Therefore, it is further demanded to increase the modulation efficiency of light with respect to an electric signal to be applied, that is, to effectively supply an electric field to an optical waveguide.
It is to be noted that the objects and application subjects of the techniques disclosed in Patent Documents 1 to 3 described above are different from those of the present invention. Therefore, even if the techniques described above are merely collected, it is difficult to solve the subject described above.